1. Field of the Invention
The present invention relates generally to an ion implantation system, and more particularly, to an ion implantation system in which the concentration of injected ions is uniform within the target.
2. Description of the Related Art
Ion implantation is a technique for implanting atomic ions into a target by imparting enough energy to the ions for them to penetrate the surface of a target, such as a silicon wafer.
Typical ion implantation systems in a semiconductor device manufacturing line are capable of regulating the concentration of an ion impurity in the range of 10.sup.14.about.10.sup.18 atoms/cm.sup.3. This is an improvement over previous methods such as ion concentration regulation through diffusion. Ion implantation systems are widely used, particularly as the level of integration of semiconductor devices increases, due to their ability to inject the ion impurities to a precise depth.
In general, conventional ion implantation systems comprise a number of components, including an ion source, an ion mass analyzer, and an ion accelerator. At each of these components, power of various levels is applied. Ions generated from an ion source containing source gas are accelerated due to the applied power. The accelerated ions are injected into a wafer placed on the end station.
The voltage levels applied to respective components of the ion implantation system greatly affect the operation of the system. The voltage levels are varied based on factors such as the characteristics of the extraction, the acceleration, and the analysis. The voltage level used is an important factor in determining the dose (i.e., the applied concentration) of the ions to the wafer.
In the conventional ion implantation system, an equal level of high is direct voltage is applied to the ion extractor and the ion accelerator. These components extract and accelerate ions before injection into the target whether they are being used in a pre-acceleration system where extracted ions are accelerated before being mass-analyzed, or in a post-acceleration system, where extracted ions are accelerated after mass-analysis.
A more detailed description of the above described ion implantation systems will now be undertaken with reference to FIGS. 1a-3.
FIG. 1a is a schematic diagram illustrating a "post-acceleration" ion implantation system 100. Ions are supplied from an ion supply source 10 having impurities in a gas or solid form, and extracted by an ion-extractor 12, to which a high voltage is applied. The extracted ion beam 16 includes ions of different masses, which then enter a mass analyzer 14. Mass analyzer 14 selects ions having a specific mass.
The selected ions enter ion accelerator 18 where they are accelerated to a predetermined energy. The accelerated ion beam 16 is injected into wafer 24 mounted on end station 22.
Ion extractor 12, mass analyzer 14, and ion accelerator 18 are shown in more detail in FIG. 1b. Ion extractor 12 is connected to a variable DC power source 26, mass analyzer 14 is connected to a power source 28, and ion accelerator 18 is connected to variable DC power source 30. Power sources 26, 28, and 30 supply power based on the weight of the ions and the desired ion implantation process. A specific power value set at any one of power sources 26, 28, and 30 corresponds to specific values at the other two power sources.
In the conventional ion implantation system 100, DC power is applied to ion extractor 12 and ion accelerator 18. A concentration distribution of ions injected into wafer 24 versus the depth of the ions in wafer 24 is shown in FIG. 3. For example, when boron ions (B+) are implanted in the wafer with an acceleration energy of 130 keV, the highest values of the concentration dose (1.9E+17 atoms/cm.sup.3) occur a certain depth into the wafer (4.0E-7 m). Ion implantation system 100 works well when it is being used to inject the highest concentration of impurities at a specific depth.
An ion implantation system 200 using the conventional "pre-acceleration method" is shown in FIGS. 2a and 2b. System 200 is similar to system 100, the primary difference being that the ion accelerator 18 is placed before the mass analyzer 14. In particular, DC power sources 26 and 30 are connected to an ion extractor 12 and an ion accelerator 18. A detailed description of implantation system 200 is omitted because of its similarity to implantation system 100.
The ion implantation process using the pre-acceleration ion implantation system 200 exhibits process characteristics similar to those of post-acceleration ion implantation system 100. That is, the graph of FIG. 3 applies to ion implantation system 200 as well as ion implantation system 100.
Conventional attempts to implant a uniform concentration of impurities at a specified depth range often did not produce satisfactory results because, as shown in FIG. 3, the peak ion concentration occurs in a very narrow positional range in the wafer. To achieve uniform concentration at a specified depth range the implantation process was typically performed repeatedly with the acceleration energy varied in each process.
As described above, the conventional ion implantation process for implanting a specific dose throughout a certain range of depths required varying the acceleration energy values used in the implantation process and repeating the ion implantation process, resulting in wasted process time and decreased productivity.